1. Field of the Invention
This invention relates to a spectacle frame, and in particular to a plastic spectacle frame.
2. Description of the Prior Art
Spectacle frames provided with a plastic lens bow have been gaining public favor for their light weight and diversity of design. The conventional plastic lens bow is formed in a closed ring-like shape, and when lenses are to be mounted thereon, the diameter of the rims thereof is expanded by heating the lens bow and after the lenses have been mounted thereon, the lens bow is cooled to contract the rims to their original diameter.
Now, plastics are weaker in strength than metals and therefore, it is necessary to make the lens bow thick, and this leads to a disadvantage that the lens bow formed of plastic narrows the field of view and is inferior in appearance. For this reason, investigation of plastic materials having great strength has progressed and recently, frames having a lens bow using plastics reinforced by carbon fibers have become commercially available. In the case of this lens bow, the plastics reinforced by carbon fibers are tough even if they are thin, but they hardly expand when heated and accordingly do not permit the conventional lens mounting method to be adopted. So, each rim of the lens bow has been formed as a substantially C-shaped open ring and a pair of integrally formed coupling portions have been provided at the ends of the open ring, and the two coupling portions have been fastened together by a metallic self-tapping screw to thereby close the bow and hold lenses between the coupling portions.
However, the use of the self-tapping screw may destroy the threads during the mounting or interchange of lenses because the internal thread is formed in the plastic coupling portion. Particularly, Nylon material reinforced by carbon fibers softens in its moisture absorbing state and therefore, the threads are easily destroyed.
On the other hand, metallic hinges are used as hinges for coupling the lens bow to temples. Because a great force is applied to the hinges, the plastic coupling surface is liable to be worn away by frequent opening and closing of the hinges. Moreover, it is desired to adjust the angle of inclination of the temples by bending the hinges. In that case, the hinges are partially mounted on one of the plastic coupling portions by insert molding.